The broad aim of this research is to contribute to our understanding of nucleic acid enzymology. We propose to approach this goal by studying and exploiting one particular enzyme, T4 RNA ligase, which catalyzes reactions involving nucleic acids. Our specific long-term objectives are: 1) to arrive at a detailed understanding of the reaction characteristics, polynucleotide substrate specificities, and catalytic mechanisms of action of the enzyme and 2) to capitalize on the enzyme's unique synthetic capabilities by constructing the specific DNA recognition sequences and its analogs will be used to probe the molecular nature of the specific protein-nucleic acid interaction which occurs between these enzymes and their substrates. Another goal is to continue to examine and develop RNA ligase as a nucleic acid modification tool by synthesizing substrates which can be used to label the 3' -ends of RNA and DNA.